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ShellCheck – Static Analysis Tool for Shell Scripts

ShellCheck is a static analysis tool that gives warnings and suggestions for bash/sh shell scripts.

How To Use ShellCheck

There are a number of ways to use ShellCheck.

  • On The Web
Paste a shell script on https://www.shellcheck.net for instant feedback.
ShellCheck.net is always synchronized to the latest git commit, and is the easiest way to give ShellCheck a go.
  • From Your Terminal
Run shellcheck yourscript in your terminal for instant output, as seen above.
  • In Your Editor
You can see ShellCheck suggestions directly in a variety of editors.
    • Vim, through ALE, Neomake, or Syntastic
    • Emacs, through Flycheck
    • Sublime, through SublimeLinter.
    • Atom, through Linter.
    • VSCode, through vscode-shellcheck.
    • Most other editors, through GCC error compatibility.
  • In your build or test suites
While ShellCheck is mostly intended for interactive use, it can easily be added to builds or test suites.
ShellCheck makes canonical use of exit codes, and can output simple JSON, CheckStyle compatible XML, GCC compatible warnings as well as human-readable text (with or without ANSI colors).

What kind of things does ShellCheck look for?

Here is an incomplete list of detected issues:

  • Quoting:
ShellCheck can recognize several types of incorrect quoting:
echo $1                           # Unquoted variables
find . -name *.ogg                # Unquoted find/grep patterns
rm "~/my file.txt"                # Quoted tilde expansion
v='--verbose="true"'; cmd $v      # Literal quotes in variables
for f in "*.ogg"                  # Incorrectly quoted 'for' loops
touch $@                          # Unquoted $@
echo 'Don't forget to restart!'   # Singlequote closed by apostrophe
echo 'Don\'t try this at home'    # Attempting to escape ' in ''
echo 'Path is $PATH'              # Variables in single quotes
trap "echo Took ${SECONDS}s" 0    # Prematurely expanded trap
  • Conditionals:
ShellCheck can recognize many types of incorrect test statements.
[[ n != 0 ]]                      # Constant test expressions
[[ -e *.mpg ]]                    # Existence checks of globs
[[ $foo==0 ]]                     # Always true due to missing spaces
[[ -n "$foo " ]]                  # Always true due to literals
[[ $foo =~ "fo+" ]]               # Quoted regex in =~
[ foo =~ re ]                     # Unsupported [ ] operators
[ $1 -eq "shellcheck" ]           # Numerical comparison of strings
[ $n && $m ]                      # && in [ .. ]
[ grep -q foo file ]              # Command without $(..)
[[ "$$file" == *.jpg ]]           # Comparisons that can't succeed
(( 1 -lt 2 ))                     # Using test operators in ((..))
  • Frequently misused commands:

ShellCheck can recognize instances where commands are used incorrectly:

grep '*foo*' file                 # Globs in regex contexts
find . -exec foo {} && bar {} \;  # Prematurely terminated find -exec
sudo echo 'Var=42' > /etc/profile # Redirecting sudo
time --format=%s sleep 10         # Passing time(1) flags to time builtin
while read h; do ssh "$h" uptime  # Commands eating while loop input
alias archive='mv $1 /backup'     # Defining aliases with arguments
tr -cd '[a-zA-Z0-9]'              # [] around ranges in tr
exec foo; echo "Done!"            # Misused 'exec'
find -name \*.bak -o -name \*~ -delete  # Implicit precedence in find
f() { whoami; }; sudo f           # External use of internal functions
  • Common beginner’s mistakes:

ShellCheck recognizes many common beginner’s syntax errors:

var = 42                          # Spaces around = in assignments
$foo=42                           # $ in assignments
for $var in *; do ...             # $ in for loop variables
var$n="Hello"                     # Wrong indirect assignment
echo ${var$n}                     # Wrong indirect reference
var=(1, 2, 3)                     # Comma separated arrays
array=( [index] = value )         # Incorrect index initialization
echo "Argument 10 is $10"         # Positional parameter misreference
if $(myfunction); then ..; fi     # Wrapping commands in $()
else if othercondition; then ..   # Using 'else if'
  • Style:
ShellCheck can make suggestions to improve style:
[[ -z $(find /tmp | grep mpg) ]]  # Use grep -q instead
a >> log; b >> log; c >> log      # Use a redirection block instead
echo "The time is `date`"         # Use $() instead
cd dir; process *; cd ..;         # Use subshells instead
echo $[1+2]                       # Use standard $((..)) instead of old $[]
echo $(($RANDOM % 6))             # Don't use $ on variables in $((..))
echo "$(date)"                    # Useless use of echo
cat file | grep foo               # Useless use of cat
  • Data and typing errors:
ShellCheck can recognize issues related to data and typing:
args="$@"                         # Assigning arrays to strings
files=(foo bar); echo "$files"    # Referencing arrays as strings
declare -A arr=(foo bar)          # Associative arrays without index
printf "%s\n" "Arguments: $@."    # Concatenating strings and arrays
[[ $# > 2 ]]                      # Comparing numbers as strings
var=World; echo "Hello " var      # Unused lowercase variables
echo "Hello $name"                # Unassigned lowercase variables
cmd | read bar; echo $bar         # Assignments in subshells
  • Robustness:
ShellCheck can make suggestions for improving the robustness of a script:
rm -rf "$STEAMROOT/"*            # Catastrophic rm
touch ./-l; ls *                 # Globs that could become options
find . -exec sh -c 'a && b {}' \; # Find -exec shell injection
printf "Hello $name"             # Variables in printf format
for f in $(ls *.txt); do         # Iterating over ls output
export MYVAR=$(cmd)              # Masked exit codes
  • Portability:
ShellCheck will warn when using features not supported by the shebang. For example, if you set the shebang to #!/bin/sh, ShellCheck will warn about portability issues similar to checkbashisms:
echo {1..$n}                     # Works in ksh, but not bash/dash/sh
echo {1..10}                     # Works in ksh and bash, but not dash/sh
echo -n 42                       # Works in ksh, bash and dash, undefined in sh
trap 'exit 42' sigint            # Unportable signal spec
cmd &> file                      # Unportable redirection operator
read foo < /dev/tcp/host/22      # Unportable intercepted files
foo-bar() { ..; }                # Undefined/unsupported function name
[ $UID = 0 ]                     # Variable undefined in dash/sh
local var=value                  # local is undefined in sh
time sleep 1 | sleep 5           # Undefined uses of 'time'
  • Miscellaneous:
ShellCheck recognizes a menagerie of other issues:
PS1='\e[0;32m\$\e[0m '            # PS1 colors not in \[..\]
PATH="$PATH:~/bin"                # Literal tilde in $PATH
rm “file”                         # Unicode quotes
echo "Hello world"                # Carriage return / DOS line endings
echo hello \                      # Trailing spaces after \
var=42 echo $var                  # Expansion of inlined environment
#!/bin/bash -x -e                 # Common shebang errors
echo $((n/180*100))               # Unnecessary loss of precision
ls *[:digit:].txt                 # Bad character class globs
sed 's/foo/bar/' file > file      # Redirecting to input

Installation:

The easiest way to install ShellCheck locally is through your package manager.

  • On systems with Cabal (installs to ~/.cabal/bin):
cabal update
cabal install ShellCheck
  • On systems with Stack (installs to ~/.local/bin):
stack update
stack install ShellCheck
  • On Debian based distros:
apt-get install shellcheck
  • On Arch Linux based distros:
pacman -S shellcheck
  • On Gentoo based distros:
emerge --ask shellcheck
  • On EPEL based distros:
yum -y install epel-release
yum install ShellCheck
  • On Fedora based distros:
dnf install ShellCheck
  • On OS X with homebrew:
brew install shellcheck
  • On OS X with MacPorts:
port install shellcheck
  • On openSUSE:Tumbleweed:
zypper in ShellCheck
  • On other openSUSE distributions:

Add OBS devel:languages:haskell repository

zypper ar http://download.opensuse.org/repositories/devel:/languages:/haskell/
openSUSE_$(version)/devel:languages:haskell.repo
zypper in ShellCheck
  • On Solus:
eopkg install shellcheck
  • From Docker Hub:
docker pull koalaman/shellcheck:latest  # Or  :v0.4.6  for a release version
docker run -v "$PWD:/mnt" koalaman/shellcheck myscript

or use koalaman/shellcheck-alpine if you want a larger Alpine Linux based image to extend.

Alternatively, get freshly built binaries for the latest commit here:
or see the storage bucket listing for checksums and release builds.

Travis CI Setup

If you want to use ShellCheck in Travis CI, you can most easily install it via apt:
language: bash
addons:
  apt:
    sources:
    - debian-sid    # Grab ShellCheck from the Debian repo
    packages:
    - shellcheck

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